Image quality test results have been added to the Canon EF 400mm f/2.8L IS III USM Lens page.
Is the Canon EF 400mm f/2.8L IS III Lens sharper than the EF 400mm f/2.8L IS II? It is hard to tell the difference in a perfection vs. perfection comparison. That the III weighs so much less than the II makes the III's image quality look much better to me.
The Canon EF 400mm f/2.8L IS III USM Lens is in stock at B&H and available for preorder at Amazon | Adorama | WEX.
Rent the Canon EF 400mm f/2.8L IS III USM Lens from Lensrentals.